1. Field of the Invention
The present invention relates to an ink composition that is preferably used for inkjet recording, an image forming method and a printed article produced by the image forming method. The invention relates, particularly, to an ink composition preferable for inkjet recording, the ink composition being superior in the dispersibility of a colorant so that it has excellent color developing ability, being cured by irradiation with an active energy ray and enabling the formation of a high quality image, an image forming method and a printed article produced by this image forming method.
2. Description of the Related Art
Electrophotographic systems, sublimation-type and melting-type thermal transfer systems, and inkjet systems are image recording methods of forming an image on a recording medium such as paper based on image data signals. In particular, inkjet systems are applicable to an inexpensive apparatus, and perform direct image formation on a recording medium by ejecting ink only to image areas where ink deposition is necessary; therefore, inkjet systems use ink effectively, thus reducing the running cost. Further, inkjet systems generate less noise and are superior image recording systems.
Inkjet systems enable printing on recording media that do not absorb water, such as plastic sheets and metallic plates, as well as on paper. In inkjet systems, it is an important target to achieve higher printing speed and higher printing quality, and the time that liquid droplets take to dry and cure after printing has significant influence on the sharpness of the image. In one inkjet system, an inkjet recording ink that can be cured by irradiation with an active energy ray is used. According to this system, sharp images can be formed by curing ink droplets by irradiation with an active energy ray just after printing.
In order to form highly accurate images with excellent coloring properties, such a curable ink composition is required to have high dispersibility of a colorant, particularly a pigment, and stability over time. Reduction in the colorant size is necessary for imparting clear tone and high tinting strength to the ink composition. In particular, ejected droplets of the inkjet ink exert a significant influence on the sharpness of images; therefore, the ejection quantity of the ink droplets needs to be small, and the size of the colorant particles contained in the ink has to be smaller than the thickness of the film formed by curing of the ink. However, when the size of the pigment particles is reduced so as to achieve higher tinting strength, the fine particles are hard to disperse, and aggregates are easily formed. Another problem is created in that the viscosity of the ink composition is increased by the addition of a dispersant. The formation of the colorant aggregates and the increase in the viscosity of the ink composition both adversely affect ink ejection property, and such an ink composition is not preferred.
When an ink composition is used in inkjet recording, the ink composition is contained in a cartridge. The ink composition in the cartridge is heated at ejection and cooled at a non-ejection time or at storage; in this way, the ink composition undergoes repeated temperature changes (heating-cooling). This temperature changes also adversely affect the colorant dispersibility, and the colorant dispersibility is deteriorated with time, whereby problems are created in that thickening and aggregation easily occurs.
Accordingly, there is a demand for an ink composition having sufficient fluidity and excellent stability of the colorant dispersion over time, in which fine colorant particles are dispersed stably. Various proposals have been made concerning dispersants for achieving stable colorant dispersion liquids.
For improving compatibility with the colorant, an ink composition using a pigment derivative as a dispersant (see e.g. Japanese Patent Application Laid-Open (JP-A) Nos. 2003-119414 and 2004-18656) and an ink composition that uses a polymer having a basic group as a dispersant for a specific pigment such as a phthalocyanine-based or quinacridone-based pigment (see e.g. JP-A No. 2003-321628) and an ink composition (see e.g. JP-A No. 2004-131589) which is free of organic solvent and contains a dispersant (e.g., a special polymer compound, poly(ethyleneimine)-poly(12-hydroxystearic acid) graft polymer) and a specific monomer that dissolves the dispersant have been proposed.
In this ink composition, the pigment dispersibility is actually improved owing to the function of the dispersant. However, the size of the pigment used in the ink composition is not sufficiently small, and there is a room for improvement of dispersibility of finer pigment particles. Further, the ink composition also has a problem in that the dispersion stability upon long-term storage or upon repeated temperature changes is still insufficient.